Recent developments in telecommunication and semiconductor technologies facilitate the transfer of growing amounts of information over wireless networks.
Short-range ultra wide band wireless networks are being developed in order to allow wireless transmission of vast amounts of information between various devices. U.S. patent application 2003/0063597 of Suzuki, titled “Wireless transmission system, wireless transmission method, wireless reception method, transmitting apparatus and receiving apparatus”, which is incorporated herein by reference, described wireless networks that each includes a base station.
Some of short-range ultra wide band wireless networks are characterized by a distributed architecture in which devices exchange information without being controlled by a central host or a base station.
FIG. 1 is a schematic illustration of two ultra wide band wireless networks (also referred to as personal access networks) 10 and 20, each including multiple devices that wirelessly communicate with each other. First network 10 includes first till third devices A-C11-13 and the second network 20 includes forth till sixth devices D-F24-26.
Each of the ultra wide band wireless networks uses time division multiple access (TDMA) techniques in order to allow its devices to share a single channel.
FIG. 2 illustrates a typical TDMA frame 30. TDMA frame 30 includes multiple time-slots, such as beacon slots 14 and media access slots. The media access slots include distributed reservation protocol (DRP) slots 36 and prioritized contention access (PCA) slots 38. PCA slots are also referred to as PCA periods. DRP slots are also referred to as DRP periods.
The beacon slots are used to synchronize devices to the TDMA frame 30. A typical beacon frame includes information that identifies the transmitting device. It also may include timing information representative of the start time of the TDMA frame 30.
The DRP slots 36 are coordinated between devices that belong to the same network and allow devices to reserve these slots in advance. During the PCA slots 38 devices that belong to the network compete for access based upon their transmission priority. It is noted that the allocation of media access time slots is dynamic and can change from one TDMA frame to another.
Typically, the PCA slots are assigned by applying a carrier sense multiple access with collision avoidance (CSMA/CA) scheme. If a device requests to transmit over a wireless medium it has to check if the wireless medium is idle. If the wireless medium is not idle the device has to wait a randomly determined time period. This time period is selected from a contention window that has a length that is inversely proportional to the priority of the device.
Transmission between devices that belong to the first network 10 can be subjected to interferences from devices of the second network. This can occur if, for example, a device of the second network is moved towards the devices of the first network, or if the wireless medium characteristics have changed such as to increase the transmission range of devices of the second network.
FIG. 3 illustrates a TDMA frame 30 of first network 10 as well as a TDMA frame 40 of second network 20. TDMA frame 40 includes multiple time-slots, such as beacon slots 44, DRP slots 46 and PCA slots 48.
TDMA frame 30 and TDMA frame 40 are not aligned to each other. In addition, the partition between various slots differs from the TDMA frame 10 to TDMA frame 40.
The differences between the two TDMA frames can cause transmission failures. These failures can occur PCA slots and even during DRP slots.
The wireless medium can be utilized for transmission of variable-rate streaming applications. Application rates peaks can cause a reduction in the performance of the ultra wide band network, due to network congestion, buffers overflow, timing requirements violations and loss of packets.
There is a need to reduce the effects of inter-network interference. These is also a need to improve the utilization of the wireless medium.